Ram jet burner with aqueous injection to promote smooth burning



1953 J. w. MULLEN 11, ETAL 2,648,196

RAM JET BURNER WITH AQUEOUS INJECTION I TO PROMOTE SMOOTH BURNING Filed March l8, 1947 2 Sheets-Sheet l JAMES W. MULLEN 11 JOHN B. FENN IN V EN TORS co V Patented Aug. 11, 1953 RAM JET BURNER WITH AQUEOUS INJEC- TION TO PROMOTE SMOOTH BURNING James W. Mullen II and John B. Fenn, Richmond, Va., assignors to Experiment Incorporated, Richmond, Va., a corporation of Virginia Application March 18, 1947, Serial No. 735,452

4 Claims.

Our invention relates to methods and means for improving the smoothness of combustion within ducted burners such as those employed in ram-jets.

At the present time, relatively little is known of the factors influencing the roughness or smoothness of combustion in ram-jet combustion chambers. There is some evidence that igniter design is closely related to the smoothness of burning, and there is also reason to believe that the rough or smooth qualit of burning may be a function of the homogeneous or heterogeneous nature (both with respect to physical state and to distribution in space) of the mixture of air and fuel at the source of ignition. Rough burning has usually been observed to be characteristic of the ignition of homogeneous mixtures, but it is generally expected that improved combustion efficiencies may be had if homogeneous mixtures can be burned smoothly. Regardless of its cause, rough burning is known to be detrimental to combustion efiiciency, to thrust, and to other performance factors-not to mention destructive efiects upon structures.

It is, accordingly, an object of our invention to provide methods and means for improving the performance of ducted burners, particularly of ram-jets flying at supersonic speeds.

It is another object to provide improved methods and means for promoting smooth burning in a ram-jet.

It is a specific object of our invention to provide a means for injecting a fluid or coolant into the boundary layer of a combustion chamber.

Other objects and various further features of the invention will hereinafter be pointed out or will occur to those skilled in the art from a readin of the following specification in conjunction with the accompanying drawings. In said drawmgs:

Fig. l is a sectional view of a flying-model ram-jet incorporating features of our invention;

Fig. 2 is an enlarged fragmentary sectional view illustrating parts of Fig. 1 in greater detail; and

Fig. 3 is a sectional view of a test equipment, incorporating features of the invention and simulating flight conditions Within a ram-jet combustion chamber.

Broadly speaking, our invention contemplates the injection of a fluid into or near the boundary layer of a combustion chamber. We have found such injection to be ver noticeably effective, even for burner configurations that have previously exhibited rough combustion characterist cs. The effectiveness of injection in producing smooth burning seems to vary appreciably with the location of the point of introduction of the fluid, but in general we have found a preferred position to be upstream from the locus Oi contact of a flame front with the inner wall of a ram-jet combustion chamber. The nature of the phenomenon is by no means completely understood, but more effective results have been obtained when the injected fluid is an aqueous liquid applied as a substantially continuous film along the inner wall of the combustion chamber just upstream from where the flame front contacts the wall.

In the specific forms to be described, the combustion chamber is of generally circular crosssection, and ignition is initiated and held along the axis of the combustion chamber. The developing flame front is therefore generally conical, contacting the combustion-chamber wall in a circle downstream from the point of ignition. Aqueous fluid is preferably injected so as to form a fllm or at just upstream from this circle. It should be noted that this circle is in general not sharply defined.

Referring to Figs. 1 and 2 of the drawing, our invention is shown in application to a so-called flying-model ram-jet of a type which has been repeatedly flown at supersonic speeds. The more important components of this ram-jet include a generally ogival nose 5 embracing an inlet opening 6 of circular cross-section. Air entering the inlet 6 is expanded ina diffuser l to the full internal diameter of a combustion chamber 8. The expansion ratio of the diffuser 1 may be of the order of 3:1.

Fuel may be introduced into the internal airstream by downstream injection through an injection orifice '9 located near the throat of the diil'user 1. In passing through the diffuser l, the fuel mixes with the airstream to form a combustible mixture at the upstream end of the combustion chamber 8. At this point, a preferably centrally located igniter or flame holder 10 serves to initiate and to sustain the combustion of the air-fuel mixture. The flame spreading from the igniter or flame holder 10 is generally conical, contacting the combustion-chamber wall before exhausting at the tail end ll of the engine.

In the form shown, an outer shell l2 extends from the nose piece 5 and circumferentially about the inlet 6, about the diffuser l, and about a part of the combustion chamber 8-for the purpose of providing a substantial annular space for the accommodation of fuel, metering devices, radio components, and the like. Fuel is stored in a compartment extending longitudinally between a pair of bulkheads l3-l4. In the form shown, the fuel tank includes a collapsible side or bag l5, whereby fuel may be injected by the application of gas" or other pressures upon the outside of the bag 15. Fuel-injection pressures may be applied to the space outside the bag I5 by a system of air conduits lfiextending preferably just ahead of the nose of the projectile, whereby the rate of fuelinjection is related to the air stagnation pressure. Within the fuel tank I5 one or more perforated pipes or gutters I5 may extend longitudinally and communicate with the injection means 9; the gutters I5 are. shown to be carried by the outer surface of' the diffuser I and they will be understood to perform the function of assuring steady fuel flow regardless of buckling of the fuel bag I5i If desired,.metering devices (not shown) may be employed to assure more positive control of the rate of fuel flow. To complete the identification of those parts of the projectile of Fig. 1 which are known and which therefore form no part of the present invention, thetail end II of" the projectile includes a. plurality offl'n surfaces IT, for aerodynamic stability.

In accordance with the invention, we' provide for greater assurance thatthe combustion in the ram-jet of'Figs'. I and 2 will be smooth by injecting a fluid, which is preferably aqueous, effectively in the form of a film along the combustion chamber wall upstream from the locus of contact of the flame front therewith. This injection may be accomplished by means of a suitable manifold system extending annularly about the combustion chamber and communicating with the inner surface thereof by way of a plurality of relatively small apertures I8, which may be drilled at circumferentially spaced points around the combustion-chamber wall. It will be understood that the effect of the relatively high flow rate (of the order of 200 or more feet per second) of the air-fuel mixture past the apertures I8 may be to spread the injected fluid relatively uniformly about the combustion chamber wall and that by the time such spread fluid reaches the point of flame-impingement on said wall the fluid may in effect present a circumferentially continuous film. In the form shown, the igniter or flame holder is of the so-called flared-skirt design, employing at the mouth of the flare I an expanding frusto-conical skirt I 9. The apertures I8 for introduction of the fluid or aqueous solution may be located just downstream from the lip of the skirt I9 and are preferably upstream from the locus of contact of the flame front with the combustion-chamber wall.

In order that the fluid or aqueous solution may be fed to the combustion-chamber wall at a rate roughly proportional to the fuel flow, we provide a tank 20 for the fluid to the injected. The tank 20' may have a side 2I of collapsible material for functioning in a manner analogous to the method of fuel injection which has been described. The air stagnation pressure available in the tubing I6 and present on the outer surface of the fuel bag I may also be applied to the outside of the bag 2 I. In the form shown, the bulkhead I4 separates the annular chambers in which fuel and the injectant are stored. It is, therefore, only necessary to provide for continuous air communication between these two compartments in order to have the stagnation pressure regulate both the flow of the injected fluid and of fuel. In the form shown, a plurality of apertures 22 in the bulkhead I4 and a preferably perforated gutter member 23 extending the length of the fuel tank permit such regulation. It will be understood that the function of the gutter 23 is to prevent such a buckling of the fuel bag I5 (e. g. during periods ofacceleration) as. will completely block off the availability of stagnation pressure over the tank 20. Further perforated gutters 2I may extend longitudinally within the tank 20 and communicate with the injection mean I8 so as to assure steady flow of injected fluid regardless of buckling of the bag 2-I. Again, as in the case of fuel control, metering devices (not shown) may be employed to assure more positive regulation of flow of injected fluid.

In the particular case to be discussed in con nection with Fig. 3, we prefer that the rate of flow of injected fluid (in the case of aqueous fluids) be controlled at somewhat less than six per cent of the total mass-flow of air and fuel within the ram-jet. In the form shown, this proportioning of flows may be accomplished by a suitable proportioning of the total area of the orifice or orifices available for fuel injection to the combined area of the injection orifices for the injected fluid.

Referring now to Fig. 3, we illustrate an experimental apparatus in which the nature of combustion in a flying ram-jet may be studied upon the ground. The apparatus is shown to include a substantially constant-diameter pipe 30, which serves to simulate a ram-jet combustion chamber, and air is fed into this pipe in the direction indicated by the arrow 3I and at a rate corresponding to the air mass-flow occurring at the intended flight speeds.

In the experiments which have been conducted with: the apparatus of Fig. 3, fuel has been injected at a point so far upstream from the igniter or flame holder 32 that it may be fairly assumed that mixtures passing the igniter or flameholder 32 were substantiall homogeneous. The igniter or flame holder 32 was a simple central pilot terminated at the downstream end by an outwardly flaring cone or fishtail 33. Pilot fuel consisted of a coaxial flow of oxygen and hydrogen fed through suitable supply pipes 3435. The entire pilot assembly 32 was supported by a pair of streamlined struts 36, through which the supply pipes 34-35' extended. Ignition was by means of a spark electrode 31, arcing periodically to the skirt of the fishtail 33. The flame front initiated and sustained by the pilot or flame holder 32 spread in a diverging cone to juncture with the inner surface of combustion chamber wall, as'on a circle identified by the reference numeral'38.

It should be remarked at this point that the structure thus far described in connection with Fig; 3 had consistently exhibited poor combustion, characterized principally by a roughness of burning that was accompanied by severe detonations. It is possible that these detonations may have been initiated in the boundary layer, that is, immediately adjacent the inner surface of the combustion-chamber wall. It may be that our method of injecting a fluid along the walls of the combustion chamber produces such a material change in the nature of this boundary layer as to reduce its tendency to initiate rough burning or detonations. A possible explanation for the observed effect'may be that the turbulent boundary layer no longer contains detonable or even combustible mixtures.

Whatever the reason for the improved performance, it has been definitely established that injection of a fluid, such as an aqueous liquid, along the walls of the combustion chamber upstream from the circle 38 results in material improvement of burning characteristics. For the configuration shown in Fig. 3, the most efficiently produced improvement in burning resulted when the liquid was injected through a plurality of circumferentially spaced apertures 39, located between a point one to two duct diameters downstream from the skirt or trailing edge of the pilot fishtail 33 and a point two to three duct diameters upstream therefrom. The apertures 39 were fed by an annular manifold 40 supplied by suitable piping 4|.

As an indication of the magnitude of the improvement realized with the structure as shown in Fig. 3, two typical experiments will be described.

In one experiment, to which we shall refer as Example I, the uniform circular duct 30 was 1.875 inches in diameter, and through it air (exhausting into the atmosphere) was passed at a rate of 1.20 lbs/sec. (this air mass-flow realistically represents internal conditions that occur in supersonic flight). The fuel, substantially normal pentane, was sprayed into the airstream about 60 inches above the exit of the duct at a rate of 0.079 lb./sec. and was ignited by an oxyhydrogen pilot flame maintained in the wake of the skirt 33, the skirt being 0.75 inch in diameter. The base of the skirt 33 was 19 inches from the end of the duct. The burning which ensued was rough, in that frequent detonations or explosions occurred, along with a very loud roaring noise. As well as it could be measured, the static pressure just upstream from the igniter or flame holder 32 was 32.2 lbs/in. gauge. Water was then injected at a rate of 0.049 lb./sec. through the apertures 39, which were 0.016 inch in diameter and which were 43 in number. The apertures 39 were spaced on a circle located 4.25 inches upstream from the pilot skirt 33. The immediate change in the character of burning was evidenced by a cessation of the roaring noise and by the commencement of a loud but smooth hissing sound. At the same time, the static pressure (sustained by the combustion) just upstream from the igniter or flame holder 32 increased to 48.2 lbs/in. gauge.

In the other experiment, Example 11, employing the same structure as that in Example I (except for location of injection apertures 39), the air mass-flow was 1.18 lbs./sec., and fuel comprising a mixture of heptanes was introduced at a rate of 0.083 lb./sec. Burning was again rough, the static pressure just upstream from the igniter 32 being 34.0 lbs/in. gauge. A per cent water solution of sodium chloride was then injected at a rate of 0.035 lb./sec. through the apertures 39, which were in the same circular configuration but located one inch downstream from the pilot skirt 33. Again, the change to smooth burning was immediate, the static pressure (sustained by combustion) increasing to 43.9 lbs/in. gauge.

In an attempt to determine the nature of the improvement realized by our injection of a fluid along the combustion-chamber wall, we have found substantially the same improved results to be obtainable by injection of 10 and per cent solutions of NaCl or KCl in water. The use of similar salt solutions such as CaClz, NHiOH, Na3PO4, KMnOi, and KNOs resulted in making moderately smooth combustion in an inherently rough burner configuration.

To ascertain further the nature of the improvement due to fluid injection, experiments were conducted wherein both the injected fluid and the fuel were substantially homogeneously mixed with airat the igniter or flame holder 32-that' further downstream placement of the manifold 40 and of the apertures 39 leads us to prefer such a placement as will assure the appearance of a substantially continuous (circumferentially) film of injected fluid at the presumably critical zone 38.

. Of practical importance in a consideration of our method of promoting smooth burning is the effector our injection upon thrust and upon specific impulse. When smooth burning is caused by fluid injection, there seems invariably to be an increase in flame pressure, and this increase does not appear to be due merely to the additional mass represented by the flow of injected fluid. In Example II, described above in some detail, thrust calculations were made, and the fluid injection was observed to cause an increase in thrust from 154 pounds (without fluid injection) to pounds. The specific impulse per unit mass of air was observed to increase by approximately 16 per cent over that which characterized the rough-burning conditions; the specific impulse per unit mass of fuel increased by about 14 per cent; andthe specific impulse per unit total mass (air and fuel in one case, and air, fuel, and injected fluid in the other case) increased approximately 13 per cent.

It will be appreciated that we have disclosed a relatively'simple method, and means for the implementing of this method, whereby smoother and more eflicient combustion may be eflected and assured in aram-jet-particularly in a ramjet for flight at supersonic speeds. The mass of injected fluid to be carried on the flying model need be relatively small when one considers the relatively small required flow of injected fluid and also the improved performance characteristics that result. If our present tentative conclusions as to the nature of the phenomenon are correct, then the improvement results from a surface as distinguished from a volume effect so that the larger the flying burner, the less objectionable will be the problem of providing for the storage of fluid to be injected; to be more specific, the optimum rate of injected fluid flow in larger burners (say, in those of 6, 18, or more inches diameter) may be very substantially less than six per cent of the total mass-flow of air and fuel.

Although our invention has been described in connection with fluid injection upstream from the locus of contact of a flame front with a combustion-chamber wall, it will be appreciated that such procedure is for the particular purpose of assuring smooth performance with configurations which experience has shown to be inherently rough or inefficient. With such burners (utilizing the invention) an incidental advantage has been a cooling effect on the combustionchamber wall, so that, not only can structural requirements be less'stringent from the point of view of withstanding explosive or detonating pressures, but high-temperature design factors 7 may also be modified downward. Both these considerations may assume critical significance in the saving of weight to permit greater fuel loads or greater payloads.

As indicated, the cooling efiectproduced by a practice of the invention is important, and this importance may not be limited to the case of the otherwise inherently poor burner configuration. For example, some ducted burners may be considered to be inherently rather smooth and efficient performers, and in such cases temperature considerations on the combustion chamber wall may not assume critical importance until some point downstream from the locus of contact of the flame front with the wall.

it may be desirable in certain instances to inject the fluid coolant at some point downstream. from the said locus of contact, allowing the coolant to do its work only on that part of the combustion chamber where temperature considerations would otherwise be so critical; since the burner is inherently smooth, its stable performance need not be impaired.

Incidental to the ram-jet combustion improvements resulting from application of our invention is a novel test method that the invention has been found to make possible. Previously, due perhaps to rough burning and to high temperatures along combustion-chamber walls, it had not been feasible optically to study a developing flame front of the ram-jet type; glass and other transparent structures proved unable to withstand these conditions for sufliciently long periods. Now, by injecting fluid in accordance with the invention, complete optical access to the flame is possible with glass combustion chambers. Of course, when the flow of coolant is stopped, the glass may disintegrate-but this disintegration does not occur so rapidly as to prevent a study of transitions from smooth to rough burning. The invention thus provides a tool for an examination into the nature of detonations and rough burning as they occur in ram-jets.

While we have described preferred methods and structures for carrying out these methods, it will be understood that. modifications may be made within the scope of the invention as defined in the claims which follow.

We claim:

1. In a ram-jet, an air inlet, a diffuser communicating with said inlet, a combustion chamber communicating with. said diffuser, a flame holder within said' combustion chamber, a flrst fluid tank and injector means connected thereto,

said injector means being located upstream from said flame holder, pressurizing means for said tank to assure injection of fuel therefrom, a second fluid tank and injector means connected thereto, said second-mentioned injector means including circumferentially extending means extending around the inner wall of said combustion chamber for circumferentially continuously laying an aqueous film onsaid inner wall at a location-upstream from the expected locus of flamecontact with said wall, and pressurizing means for said second fluid tank to assure injection of fluid from' said second tank into said combustion chamber, whereby said second-mentioned injector means may be effective to apply fluid from said second tank upon the wall of. said combustion' chamber at a point upstream from the ex!- pectedlocusof. contact of. a flame front on said.

well.

In order, then, to practice the invention with greater economy.

2. A ram-jet according to claim 1, in which said first and second pressurizing means are the same.

3. In a ram-jet, a generally cylindrical combustion chamber substantially open at both the upstream and the downstream ends thereof, whereby in high-speed flight there may be a very substantial flow of air through said chamber, a flame-holder spaced from the walls of said combustion chamber, fuel-injection means upstream from said flame-holder, whereby the fuel may be substantially homogeneously mixed with the substantial air flow prior to ignition of the mixture, fluid-injection means including a tank having a resiliently collapsible portion, whereby upon application of pressure said tank may be collapsed, said fluid-injection means including manifolding means extending effectively continuously around said combustion chamber and having a plurality of openings to the inner surface of the combustion chamber, said openings being located upstream from the expected locus of contact of a flame front upon the inner surface of the combustion chamber, whereby upon pressurizing an aqueous fluid in said tank during flight a film of aqueous fluid may be laid on the inner surface of the combustion chamber at the locus of flame-contact with said inner surface.

4. In a ram-jet, a generally cylindrical combustion chamber, an igniter generally coaxial with said combustion chamber, fuel-injection means, and means for injecting aqueous fluid in a film along the combustion-chamber wall upstream from the expected juncture of a flame front with the combustion-chamber wall, said last-mentioned means including an annular tank having a wall collapsible under pressure'and having a plurality of relatively closely spaced openings to the inner surface of the combustion chamber, said openings being spaced circumferentially of the combustion chamber, and means for applying pressure to said collapsible wall, whereby a fluid in said tank may be pressure fed through said openings.

JAMES W. MULLEN II. JOHN B. FENN.

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